Based on the acousto-optic effect,this paper presents a new coherent detection method with a same source and a common beam.It describes the characteristics of diffraction beam for coherent optical detection system.And it analyzes the characteristics of coherent detection by using a Gaussian spot and a flattened spot respectively.Given the conditions and methods of realizing this system,it obtains the homogenization and the coaxial relations between the two spots,which can verify the feasibility of this method.The experiments prove that there are advantages in the communication detection system by using the acousto-optic interaction and coherent detection technology.With the effect of homogenized spot,the effectiveness of optical coherent detection can be improved,which can make the receiver performance more reliable.